There is known a controlled multidigit resistance box, wherein each digit (decade) comprises ten series-connected resistors. The digits (decades) of the resistance box are interconnected in series via switches. The common lead-out of the switch of each previous decade is connected to the input of the next decade; the contacts of the resistor switch of each decade are connected to any lead-out of each resistor of the decade. The mechanical contact switches may be controlled by a drive coupled to a control unit. A certain position of the switches in each decade corresponds to a specified resistance value.
However, the use of mechanical contact switches in the resistance box under review affects the operating speed of the box and the accuracy of setting a specified resistance value. In addition, the use of such switches necessitates the use of resistors of a high grade of accuracy in each decade.
It is an object of the present invention to increase the operating speed of the resistance box.
It is another object of the invention to provide a resistance box which would ensure a high accuracy of setting a specified resistance value.
It is still another object of the invention to provide a resistance box, wherein resistors of a high grade of accuracy would not be a necessity.
The present invention essentially consists in providing a controlled multidigit resistance box, wherein each digit comprises "n" series-connected resistors and a control unit for selecting a desired combination of resistors, to switching elements of each digit, the resistance box being characterized, according to the invention, in that each digit includes an operational amplifier, and in that each lead-out of any resistor of the "n" series connected-resistors of a digit is connected via one of the switching elements to an output of the operational amplifier, and via another switching element, to the operational amplifier's inverting input, a center tap of power sources of the operational amplifier of the previous digit being connected to the input of the next digit of the resistance box.
Each digit of the resistance box may include a circuit for compensating parametric variations of the resistors.
It is highly desirable that each circuit for compensating parametric variations of resistors should include "n+1" parallel branches, each comprising a potentiometer and a switching element placed in series, and each being interposed between the output of the operational amplifier and its non-inverting input which is coupled via a resistor to the center tap of the operational amplifier's power sources.
The proposed design accounts for an icreased operating speed of the resistance box and a greater accuracy of setting a desired resistance value; as a result, the resistance box according to the invention can be used as a rapid-action standard resistance device for multichannel measuring systems.